The present disclosure relates in general to a method and apparatus of combustion incorporating a burner nozzle for burning pulverized fuels, such as low quality pulverized coal. More particularly, the present disclosure is directed to affecting a more stabilized flame while reducing nitrogen oxides, during ignition and combustion for low quality pulverized coal, and will be described with reference thereto. However, it is appreciated that the present exemplary embodiment is also amenable to other like applications.
During combustion, the chemical energy in a fuel is converted to thermal heat inside the furnace of a boiler. The thermal heat is captured through heat-absorbing surfaces in the boiler to produce steam. The fuels used in the furnace include a wide range of solid, liquid, and gaseous substances, including coal, natural gas, and diesel oil. Combustion transforms the fuel into a large number of chemical compounds. Water and carbon dioxide (CO2) are the products of complete combustion. Incomplete combustion reactions may result in undesirable byproducts that can include unburned carbon particulates, carbon monoxide (CO), and hydrocarbons (HC).
For a variety of reasons, large pulverized coal (PC) fired boners are increasingly bearing the burden of frequent load swings. The resulting variation in operating levels has increased the operation of these boilers under low load conditions. This consequently heightens the need for a burner capable of a reliable, efficient, low load performance that still enables the formation of nitrogen oxides (NOx) to be kept to an acceptable minimum level. A key factor which increases NOx formation is the oxygen available in the combustion zone immediately downstream of the burner nozzle.
Typical burner nozzles such as those described in U.S. Pat. No. 4,497,263 issued to Vatsky et al. and U.S. Pat. No. 4,457,241 issued to Itse et al. are of the type where the pulverized coal particles are concentrated into the center of an air-coal stream before these particles are burned in the boiler. This method, although sufficient for the burning of the pulverized coal, contributes to NOx formation because of the oxygen available during combustion.
Another factor influenced by burner nozzle performance is the stability of the flame. The velocity of the fuel emerging from the nozzle is of prime importance to flame stability. Lower fuel velocities provides more time for the particles to heat up and ignite in the burner throat and thereby achieves a more stable flame. Difficult to ignite fuels, such as low volatile coals, particularly benefit by lower fuel velocity. Lower velocities may also serve to limit air-fuel mixing prior to burning which reduces the availability of oxygen during combustion thereby reducing NOx formation.
Typical circular low NOx PC-fired burners have their coal nozzles positioned axially in the burner. NOx reduction is accomplished by limiting air introduction to the fuel in the near field of the flame, to reduce O2 availability during devolatilization. Limiting the rate of fuel mixing with secondary air in the near field facilitates this, and is accomplished by axial (or near axial) injection of PC into the flame. A direct consequence is that the fuel jet proceeds down the center of the flame, producing a strong fuel rich condition which persists long after devolatilization is completed. This persistent fuel rich central portion of the downstream flame delays char reactions (in absence of oxidant). Delayed char reactions are responsible for increases in unburned combustibles—unburned carbon (solid phase) and carbon monoxide (gas phase). Such increases in unburned combustibles are characteristic of many low NOx burners.
An effective solution to this problem, higher unburned combustibles with low NOx burners, is found in the AireJet® burner provided by Babcock & Wilcox Power Generation Group, Inc., which is a burner with a center air jet as disclosed by U.S. Pat. No. 7,430,970. Here, the problem is solved by adding an additional air jet supply axially to the burner, which provides an amount of oxidant to the center of the flame. This teaches supply of about 20 to 40% of the burner oxygen using the center jet, with about 10 to 30% supplied with the coal as primary air. This patent describes benefits of NOx reduction and flame stability with a burner assembly configured with an additional center air jet. Full scale results in a utility boiler indicate the AireJet® burner accomplished lower NOx and simultaneously produced low unburned combustibles at lower excess air. See technical paper titled “B&W AireJet™ Burner for Low NOx Emissions, BR-1788” which is incorporated by reference herein.
However, low quality (LQ) coals may not be directly suitable for use with AireJet® burners. Low quality coals refer to coals with excessive amounts of mineral matter (ie. ash, etc.) and moisture, often exceeding about 50% of the material. These inert materials depress the heating value of the coal, typically from about 10,000 to 12,000 to about 5000 to 7000 Btu/lb (Higher Heating Value [HHV] basis). Such LQ coals require nearly twice the mass throughput compared to higher quality coals in order to provide equivalent heat input. Consequently, twice the coal throughput requires twice the quantity of lower temperature primary air (PA) flow, typically about 130° F. to 200° F., for pulverizers to process LQ coal. This reduces the amount of high temperature secondary air (SA), typically about 600° F.-700° F. available to the burner which impairs flame stability and NOx control.
The SA/PA ratio provides an indication of relative flame stability. High SA/PA (e.g. 4) means there is proportionally more hot SA available to interact with the PA/PC jet to accelerate ignition, promote flame stability, and to influence flame development. Conversely, as SA/PA drops to a value of 2 or less, there is proportionally much less SA to influence flame development and NOx and flame stability suffer. For example, consider two coals with equal grindability but one has a heating value of 12,000 Btu/lb and one has a heating value of 6,000 Btu/lb. The SA/PA is over 4 for the 12,000 Btu/lb coal, but drops to 2 for the 6,000 Btu/lb coal. The LQ coal requires twice the PA flow on an input basis, leaving much less SA for flame control. The shortage of SA impairs implementation of AireJet® technology.
Techniques to reduce PA to the burner exist, but add costs and complexity to the process. PA can be removed by a dust separator (cyclonic or baghouse or the like), downstream of the pulverizers. Indirect firing systems employ such equipment. Such systems can fully separate PA and coal and can supply a richer PA/PC mix to the burners, at considerable expense. As an alternative, U.S. Pat. No. 4,627,366 discloses a Primary Air Exchange for a Pulverized Coal Burner and teaches the use of a burner elbow and associated apparatus to separate some PA from the PA/PC stream entering the burner (PAX burner). The separated PA, with a small amount of PC, is vented to the furnace through a pipe to a location in proximity to the burner. This effectively reduces PA to the burner, but increases costs due to associated piping, valves, and furnace wall openings. Locating this additional equipment can be problematic for wall fired boilers, and can require larger burner zones to accommodate.
LQ coals suffer from delayed ignition and poor flame stability due to massive amounts of inert material in such coals, which depress heating values of such coals. Further, the low heating value requires disproportionately high amounts of primary air to pulverize the coal, leaving lesser secondary air to shape the flame and counteract such problems.
Another known solution for this problem is disclosed by U.S. Pat. No. 4,654,001 which is also incorporated by reference and teaches a Flame Stabilizing/NOx Reduction Device for Pulverized Coal Burner, referred to as a DeNOx Stabilizer (DNS). This patent teaches a means of separating a portion of the PA entering a burner elbow and injecting it down the center of the flame. The separation device is like that used in the PAX burner, with a tubular piece concentric with the burner elbow exit capturing a portion of the PA. The concentric tubular piece then conveys this separated stream to the end of the burner and injects it into the furnace. The tubular piece may reduce in cross section as it approaches the end in order to accelerate the stream internal to the tube while decelerating the surrounding fuel rich stream. In use with high quality coals, the DNS provides improved flame stability by decelerating the main fuel jet which provides more residence time in the ignition zone. The DNS provides a richer fuel mixture such that coal devolatilization takes place with less oxidant available and thereby reduces NOx.
It is thus an object of this disclosure to provide a burner nozzle that is efficient and effective to operate with difficult to ignite fuels such as pulverized LQ coal and one which reduces NOx formation. It is another object of this disclosure to improve separation efficiency of PA from the PA/PC fuel mixture before entering into the furnace of a boiler for improved ignition performance. A further object of this disclosure is to provide a burner nozzle which increases flame stability and one which is easily capable of being retrofitted into existing burners. Another object of this disclosure is to separate the pulverized coal into a relatively fuel-dense low velocity stream and a relatively fuel-dilute high velocity stream with low pressure loss across the nozzle.